1. Field of the Invention
This invention is related to thermoelectric components and cooling devices utilizing thermoelectric semiconductor elements. In detail, it is related to the technology which enables thermoelectric elements to last longer while preventing the cooling efficiency from decreasing.
2. Description of the Prior Art
Thermoelectric components utilizing thermoelectric semiconductor elements made of such compounds as bismuth/tellurium, iron/silicon or cobalt/antimony group are used in cooling/heating devices. These thermoelectric components, where neither liquid nor gas is used, are useful as cooling/heating sources which are compact, free from rotational abrasion and require no maintenance.
Generally, in this type of thermoelectric component, two types of thermoelectric semiconductor elements, p-type and n-type, are arranged precisely. The thermoelectric semiconductor elements are soldered to electrodes and form a .pi.-type series circuit. When these thermoelectric elements and metallic electrodes are sandwiched by ceramic substrates with metallic film, they are commonly used as thermoelectric modules.
The composition of thermoelectric modules conventionally known is shown in FIG. 7, where (a) is the front view and (b) the diagonal view. As is shown in this figure, in this thermoelectric module, n-type and p-type thermoelectric semiconductor elements are positioned alternatively and attached to metallic electrodes. The metallic electrodes are attached alternatively to the top and bottom of the n-type and p-type thermoelectric semiconductor elements, and all elements are thus connected in a series circuit. In order to fix thermoelectric elements to upper and lower metallic electrodes, solder is applied. Each of the upper and lower metallic electrodes is attached to ceramic substrates which have been metalized with copper or nickel, and the entire system is thereby secured. The thermoelectric component of this type of composition is generally called a thermoelectric module.
When power is supplied to the electrodes of this thermoelectric module to allow an electric current to run from n-type element to p-type element, heat absorption (cold junction) occurs at the upper part of the .pi.-type and heat radiation (hot junction) at the lower part because of the Peltier effect. When the electrodes are connected in reverse, the directions of hot and cold junctions also switch. This phenomenon makes thermoelectric components useful in cooking/heating devices. Thermoelectric modules are of wide use; cooling LSIs, computer CPUs and laser units as well as for refrigeration.
When we use this thermoelectric component as a cooling device, we need to cool the hot junction (radiation part). Traditionally there have been two approaches to cooling thermoelectric components. One is, as shown in FIG. 8(a), the air cooling system in which Radiating Fin 32 is attached to the hot junction of Thermoelectric Component 31 so that air is sent from Fan 33 to Radiating Fin 32. The other is, as shown in FIG. 8(b), the water cooling system in which a Water Cooling Jacket 34 is attached to the hot junction of Thermoelectric Component 31 so that cooling water runs through the Water Cooling Jacket 34 in the direction of the arrows shown between points 34A and 34B.
FIG. 9 is a temperature diagram of a thermoelectric component. The parameters which evaluate the cooling capacity of the thermoelectric component are Module .DELTA.T and System .DELTA.T. Module .DELTA.T is the difference (.DELTA.T1) between the temperature at the outer edge of the cold junction of the thermoelectric component, that is, the temperature (Q2) of the upper ceramic substrate, and the temperature at the outer edge of the hot junction of the thermoelectric component, that is, the temperature (Q5) of lower ceramic substrate. System .DELTA.T is the difference (.DELTA.T2) between the temperature (Q3) of the cold-junction part of the thermoelectric component and the one (Q6) surrounding its hot-junction part. The latter perimeter temperature corresponds to the temperature around Radiating Fin 32 in FIG. 8(a) and to the temperature of water in Water Cooling Jacket 34 in FIG. 8(b).
As illustrated in FIG. 9, the lowest temperature of the thermoelectric component is measured at the edge of the cold junction of the thermoelectric semiconductor element. The temperature rises as the measuring point moves through the metallic electrode to the ceramic substrate and ends up at Q3 at the cooling load. The highest temperature (Q4) is measured at the edge of the hot junction of the thermoelectric semiconductor element. The temperature falls as the measuring point moves through the metallic electrode to the ceramic substrate and ends up at Q6 around the hot junction. Owing to a small thermo-conductivity, ceramic substrates lower the cooling efficiency significantly.
The Peltier cooling device is therefore suggested as preventing the cooling efficiency from decreasing (Nikkei Mechanical, pp 48-56, Sep. 16, 1996). In this device, aluminum substrates with the oxidized surface, are used instead of ceramic ones. It also has a water cooling jacket with a spraying nozzle which can cool the aluminum substrate at the hot junction efficiently. This Peltier cooling device is said to provide the same cooling efficiency as that of an ordinary cooling system with flon gas.
However, this Peltier cooling device and the cooling devices shown in FIG. 8, do not provide the optimum performance from thermoelectric semiconductor elements because thermoelectric semiconductor elements are cooled indirectly through a lower substrate in each of them.
The thermoelectric semiconductor elements shown in FIG. 7 receive a large amount of thermal stress during operation, owing to the stiff structure in which the elements are fixed on top and bottom to the ceramic substrates. This results in a short life of thermoelectric semiconductor elements.